1. Field of the Invention
The present invention relates to a reciprocating compressor, and particularly to an apparatus and a method for controlling an operation of a reciprocating compressor capable of improving operational efficiency of a compressor by calculating a mechanical resonant frequency corresponding to a load state, and variably controlling a frequency according to the calculated resonant frequency.
2. Description of the Background Art
In these days, compressors of various types are being used, and most generally used compressor among those is a reciprocating compressor for sucking, compressing, and discharging refrigerant gas while a piston linearly reciprocates in a cylinder.
The reciprocating compressor can be divided into a recipro type and a linear type according to a type of driving a piston.
In the recipro type compressor, a rotation movement of a motor is converted into a linear movement to compress refrigerant gas, and so in order to convert a rotation movement into a linear movement, there is a need for a converting apparatus such as a screw, a chain, a gear system, a timing belt or the like. Accordingly, energy loss generated due to the conversion is great, and a structure of an apparatus is complicated. For this reason, a reciprocating compressor of the linear type where the motor itself linearly reciprocates, is generally used in these days.
In a linear type reciprocating compressor, since the motor itself generates a linear driving force, there is no need for a mechanical converting apparatus, a structure thereof is not complicated, loss caused by conversion of energy can be reduced, and noise can be remarkably reduced since there is no coupling portion where friction and an abrasion are generated,
In case of using the reciprocating compressor of the linear type for a refrigerator and an air conditioner, since as varying a stroke voltage applied to the compressor, a compression ratio is varied, freezing capacity can be variably controlled.
FIG. 1 is a block diagram showing a structure of an apparatus for controlling an operation of a conventional reciprocating compressor.
As shown therein, the apparatus for controlling the operation of the conventional reciprocating compressor includes a reciprocating compressor 3 varying a stroke (which means a distance between a top dead center of a piston and a bottom dead center thereof) by reciprocation of a piston by a stroke voltage, and thus controlling freezing capacity (or refrigerating capacity); a voltage detecting unit 5 for detecting a voltage generated from the reciprocating compressor 3; a current detecting unit 6 for detecting a current generated from the reciprocating compressor 3; a stroke calculating unit 4 for estimating a stroke by the detected current and voltage and a motor parameter; a comparator 1 for comparing the estimated stroke value and a stroke reference value, and then outputting a difference signal according to the comparison result; and a controller 2, according to the difference signal, varying a voltage applied to the motor, and thus controlling the stroke.
Control operations of the conventional reciprocating compressor constructed as above, will now be described.
In the reciprocating compressor 3, when the compressor 3 receives a predetermined stroke reference value from a user, and then outputs a stroke value, a stroke is varied by a vertical movement of a piston in a cylinder, and a cooling gas in the cylinder is transmitted to a condenser through a discharging valve thereby controlling freezing capacity.
Herein, as the stroke is varied by the stroke voltage, a voltage detecting unit and a current detecting unit detect a voltage and a current generated from the reciprocating compressor, and output the detected voltage and current values to the stroke calculating unit 4 respectively. Accordingly, the stroke calculating unit 4 calculates a velocity of the piston (equation 1), and a stroke (equation 2), by applying the current and the voltage and a motor parameter to equation below, and thereafter, outputs the estimated stroke value to the comparator 1.
                    Velocity        =                              1            α                    ⁢                      (                                          V                M                            -              Ri              -                              L                ⁢                                                                  ⁢                                                      ⅆ                    i                                                        ⅆ                    t                                                                        )                                              (        1        )                                Stroke        =                              1            α                    ⁢                      ∫                                          (                Velocity                )                            ⁢                              ⅆ                t                                                                        (        2        )            
Herein, α means a motor constant for calculating a stroke and for converting electrical power into mechanical power, R a loss value caused by resistance such as copper loss and iron loss, L inductance, and VM a voltage between both ends of a motor.
Thereafter, the comparator 1 compares the estimated stroke value and the stroke reference value and applies a difference signal to the controller 2 according to the comparison result. Accordingly, the controller 2 varies a voltage applied to the motor of the compressor 3 and thus controls a stroke.
FIG. 2 is an operation flow chart of a method for controlling an operation according to an estimated stroke value of the conventional reciprocating compressor.
As shown therein, the stroke calculating unit 4 receives a voltage and a current applied to the reciprocating compressor, and estimates a current stroke (SP1).
Thereafter, if the estimated current stroke value is greater than the stroke reference value, the controller 2 decreases a voltage applied to a motor (SP2, SP4), or if the estimated current stroke value is smaller than the stroke reference value, the controller 2 increases a voltage applied to a motor (SP2, SP3).
However, in the apparatus and the method for controlling the operation of the conventional reciprocating compressor, even though a mechanical resonant frequency is varied as a load (a temperature of outer air of a refrigerator or a temperature of a condenser) is varied, a stroke is controlled by a constant frequency without varying an operation frequency, therefore operational efficiency of the compressor is deteriorated.